1. Field of Invention
The present invention relates to structured self-cleaning surfaces and a method of forming the same. More particularly, the method of the present invention involves growing diatoms on a surface to form siliceous nanoscale-sized structures and treating the structures with a hydrophobic agent to form the structured self-cleaning surface.
2. Description of Related Art
In order to possess a true “self-cleaning” capability, a surface must have both hydrophobic properties and a micro-rough surface structure. Nature produces such an effect in the leaves of the lotus plant. A waxy, hydrophobic substance and pyramidal elevations with spacing on the order of a few micrometers characterize the surface of the lotus leaf. When water droplets come into contact with the lotus leaf, adhesion is minimal because the leaf surface is hydrophobic and high contact angles are formed. Further, the micro-rough texture reduces the contact area between the water and the leaf surface, minimizing friction. Thus, a water droplet in contact with the leaf surface will roll off, picking up loose particulates (e.g., dust and pollen) in the process. The self-cleaning character of the lotus leaf was described by A. A. Abramzon and Khimia I Zhizu in 1982.
Even before Abramzon et al. described the self-cleaning effect of lotus leaves, others disclosed surface preparations with water-repellent effects. U.S. Pat. No. 3,354,022, for example, teaches that a water repellent surface can be created from a hydrophobic material exhibiting a micro-rough structure. In one embodiment, such a surface can be created on ceramic brick or glass substrates by coating the substrate with a suspension containing glass spheres with a diameter within the 3-12 μm range and a fluorocarbon wax based on a fluoroalkylethoxy methacrylate polymer. Although such surfaces exhibit the self-cleaning effect, durability remains a concern. Durability as used here means commercially acceptable resistance from abrasion, rain, wind, chemical substances, ultraviolet light from the sun, temperature, etc.
Other self-cleaning surfaces based on polymeric structures are taught, for example, in EP 0 909 747 A1 and EP 0 933 388 A2. These applications teach artificial surface structures comprised of elevations and depressions made from hydrophobic polymers. In particular, the distance between the elevations is within the 5 to 200 μm range (EP 0 772 514 B1) or the 50 nm to 10 μm range (EP 0 933 388 A2) and the height of the elevations is within the 5 to 100 μm range or the 50 nm to 10 μm range, respectively. The methods used to produce these self-cleaning surfaces include etching and embossing, coating processes for sticking on a structure-forming powder and shaping processes using appropriately structured female moulds. Like the previous self-cleaning inventions described, these polymeric surfaces are also likely to have low durability.
Substantial effort has been made to improve the durability of self-cleaning surfaces, but most involve costly, complicated methods that ultimately sacrifice self-cleaning capability. U.S. Pat. No. 6,291,022, for example, teaches a process for fabricating a water-repellent glass. The method first involves the preparation of a metal alkoxide-based silane solution, which is aged under acidic and basic conditions to produce a polymer of granular colloidal silica cross-linked with a linear polymer of siloxane. The aged solution is applied to a glass substrate and subjected to a thermal treatment (up to 550° C. in one embodiment). The resulting product is a densified silica layer with a fine silica particulate distribution bonded to the substrate. A water-repellent substance, such as fluoromethoxysilane, is then applied to the densified silica layer. The increased surface area and porosity of the underlying silica layer improves the durability of the water-repellant layer. However, one drawback of this method is that it does not disclose a surface optimized for the micro-roughness necessary for the self-cleaning effect.
U.S. Published Pat. Appl. No. 2003/0096083 teaches that surfaces of objects, such as containers for receiving liquid, can be modified by grit blasting or acid etching to achieve the micro-roughness necessary for self-cleaning capability. It also teaches the use of a “contour-following coating” applied to the underlying micro-rough surface. The invention is a relatively simple and flexible two-step, low temperature process capable of operating on various substrate surface geometries. One disadvantage of the invention, however, is that it may be constrained to substrate materials with sufficient toughness (metal or polymeric materials) to withstand the aggressive grit blasting step. Other substrates, such as glass or ceramic materials, may be prone to micro-cracking from the grit blasting procedure, making them susceptible to fracture or reduced transparency. Roughness on the nanometer scale is also not claimed by this method, which may limit the degree of self-cleaning capability obtained.
U.S. Pat. Appl. No. 2003/0152780 teaches the creation of a micro-rough surface comprised of glass flux and structure-forming particles in the 0.1 to 50 μm range. Good durability of the micro-rough surface is achieved in certain embodiments of the invention in which the glass flux layer is sintered at high temperature to glass or ceramic substrate materials. Self-cleaning capability is obtained with the addition of a thin, hydrophobic coating. In one embodiment, reactive alkyl or preferably fluoroalkyl silanes and oligomeric alkyl or fluoroalkyl siloxanes are cured over the glass flux layer, providing a hydrophobic coating a few nanometers thick. Rather than relying on adhesion, this cured layer has improved durability because it is chemically bonded (Si—O—Si bridge) to the glass flux-coated substrate. In practice, this invention may suffer from less than optimal durability and micro-roughness as the hydrophobic layer is relatively thin and the structure-forming particles exceed 0.1 μm in size.
Although progress has been made in the development of self-cleaning industrial surfaces and coatings, inventive processes producing an optimal combination of high durability, self-cleaning capability, flexibility and cost efficiency remain elusive. Self-cleaning coatings are often used on automobile windshields for improved poor weather visibility. But the methods currently employed are limited in durability and require frequent application of the self-cleaning coating product. Similarly, the costs of cleaning industrial building windows, particularly those in multi-story buildings, and the windows of cruise ships could be dramatically reduced if a self-cleaning substrate surface were available with high abrasion resistance. Thus, the need for a highly structured, durable, nano-rough surface remains important.
Diatoms are a large group of single-celled algae species that possess a highly ornate, porous and intricate skeleton (frustule), comprised of silica, alumina and other substances. The structure of the frustules varies significantly between various diatom species. The typical size of frustules ranges from 0.75 to 1,000 μm with nanoscale-sized features. The frustules are very durable when one considers their size, and are often found intact after thousands of years in the millions of tons of diatomaceous earth sediments found on the ocean floor.
With such diversity in microscopic features, porosity and durability, it should be no surprise that a significant body of prior art has taught the use of diatoms for use in filtration applications. Many particle separation methods rely on diatomite products in the processing of beer, for example. Diatomite is also used as filler in paints and paper. U.S. Published Pat. Appl. No. 2001/0023233 teaches the purification of diatomaceous earth to produce a highly pure, silica particulate. The resulting product can be used in various filtration applications and retains the structural features of the diatom frustules.